Maximum 'SEO Safe' Number of Sub Domains

There is a nice thread over at Search Engine Watch Forums named Subdomain Limit Per Domain. The thread innocently asks, what is the a safe number of sub domains to put on a single domain name? As many of you know, sub domains from old domains are thriving these days. So, even more reason to ask the question, is 20 sub domains on a domain too much? Does it make the domain too suspicious? Does it raise a red flag?

In my opinion, and how I stated it in the thread, there is no magic number. I suspect it has to do a lot with the structure of the site. Including the following variables:

Number of pages on the domain

Number of pages within a particular sub domain

The primary site navigation

The secondary site navigation

Linkage data towards the primary and secondary navigation

And the number of subdomains based on the above

Those are just some of my guesses. Like anything, it is about your "intent." If the intent is to rank pages well and not serve the end user, then something may be wrong. If you are like about.com, with a trillion sub domains, but they are very suiting for the user's reader, then it is fine.